With several weeks or more remaining in a particularly deadly influenza season, U.S. health officials on Thursday urged flu vaccinations for everyone over the age of 6 months, including pregnant women.

This year's illnesses have been caused predominantly by the H1N1 virus -- the same "swine flu" strain that caused the pandemic in 2009.

Although officials say the prevalence of flu-related illness this season appears to be lower than in 2009, preliminary reporting suggests that it has killed or hospitalized more young and middle-aged adults than usual -- particularly those with existing health problems.

In studies released Thursday, CDC epidemiologists said that based on a national sampling of 122 U.S cities, 61% of all flu-related hospitalizations involved adults between the ages of 18 and 64. In the past several years, this rate has been much lower -- between 35% to 43%.

And, in a report that focused only on California, the state's department of public health reported that 405 people younger than age 65 had either died or been admitted to an intensive care unit due to the flu between Sept. 29 and Jan. 18. This was more than any full season since the pandemic.

Of the 94 deaths examined in that California report, 85% of the cases involved individuals with pre-existing conditions that made them extremely vulnerable to health complications, authors wrote. Those conditions included diabetes mellitus, chronic obstructive pulmonary disease, asthma and morbid obesity.

While it remains unclear exactly why hospital admissions have increased among young and middle-age people, officials note that they are traditionally the least likely people to seek annual flu vaccinations.

Health officials estimate that only 34% of people between 18 and 64 get annual flu shots, while the vaccination rate for people over 65 is 62%. The vaccination rate for youths aged 6 months to 17 years is 41%.

"This season is hitting middle-age and younger adults hard and the season is likely to continue for some time," Frieden told reporters. "It's not too late to get a flu shot."

Each year, health officials formulate a vaccine that will address those viral strains that are expected to be most active. In this case, they accurately predicted the return of H1N1 and included it in the vaccine.

Although officials insist vaccination is the best, first-line defense against influenza, they acknowledged that some people who get the vaccine will still get ill. (At least six of the fatal flu cases included in the California report involved people who had received vaccinations.)

"This year's flu vaccine is working," Frieden said. "It's not working as well as we wish it would."

In reports released Thursday, CDC officials estimated that the vaccine was 62% effective.

"That means if you were vaccinated, you are quite likely to be protected from the flu viruses that have been circulating this season," said Dr. Anne Schuchat, assistant surgeon general and director of the CDC's National Center for Immunization and Respiratory Diseases.

"There's a lot of myths and misinformation out there," Schuchat said. "It doesn't cause flu, but some people worry that it might ... the CDC recommends that everyone 6 months and older get a flu vaccine and that includes pregnant women."

In cases where people experienced serious flu symptoms, whether they were vaccinated or not officials urged the use of antiviral medications, which are prescribed by a doctor.

"For people who are sick with the flu, the California analysis has a very important message for doctors," Frieden said. "If they're very sick, don't wait for a test, don't not treat if the test is negative. If you think clinically it might be flu, treat for flu promptly. Unfortunately, we're seeing that only a small proportion of people who are severely ill ... are treated for flu promptly."

[For the Record, 3:10 p.m. Feb. 20: An earlier version of this post reported incorrectly that the vaccination rate for people over the age of 65 was 95%. It is 62%.]